Several methods for preventing unauthorized copying of image data exist. These methods are usually based on locating fixed patterns or indicia in the image being copied. Alternatively, some prior art have indicia that encode data messages, where the indicia have alignment or synchronizing marks for detection in a decoder. Other prior art detects several 1-dimensional barcodes on a page.
Additionally, methods exist for embedding and detecting more than one watermark included in an image, in order to determine authenticity of an image. These methods involve embedding multiple watermarks, which have different characteristics, onto an image. The watermark characteristics are chosen such that the watermarks will be affected in different manners if the image is subsequently copied. This method is limited to distinguishing original documents from copies and does not lend itself to detecting arbitrarily placed watermarks on an image—as is common with copy attacks.